A quantum cascade laser (QCL) is a multilayer semiconductor laser, based only on one type of carriers (usually electrons). A schematic diagram of a typical QCL is shown in FIG. 1. It consists of multiple layers of InxGa1-xAs/AlyIn1-yAs/InP having different compositions x and y, typically grown using molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD) techniques. Electric current in these devices is injected along the x-axis, perpendicular to the grown layers. An insulator confines the current under the contact stripe, preventing it from spreading in the y-direction. When carriers reach the gain section, they emit photons through intersubband radiative transitions (see below). Waveguide and cladding layers confine emitted light around the gain region and direct it along the z-axis. A laser of this type is described in U.S. Pat. No. 5,457,709 and Faist et al., “Quantum Cascade Laser”, Science, vol. 264, pp. 553-556, (Apr. 22, 1994).
The gain section of a QCL usually consists of twenty (20) to sixty (60) identical gain stages. A gain stage consists of approximately twenty (20) very thin InxGa1-xAs and AlyIn1-yAs layers (1-5 nm) with alternating bandgap values (quantum wells and barriers, respectively). Many alternative systems have also been demonstrated. A schematic of the conduction band diagram of one gain stage under an applied electric field is shown in FIG. 2. In an ideal case each carrier emits one photon in each gain stage.
As described in U.S. Pat. No. 5,457,709, the layers within the stage are usually divided into two regions: the active region and the energy relaxation region (injector). The active region is designed for light emission through carrier radiative intersubband transitions (transition from level 3 to level 2 in FIG. 2), while the energy relaxation region (injector) is used for energy relaxation of carriers before injection into the next stage.
Carrier population inversion between the upper and lower laser levels (levels 3 and 2 in FIG. 2), required for lasing, can be achieved when the upper laser level lifetime, τ3, is longer than the lower laser level lifetime, τ2. As claimed in U.S. Pat. No. 5,457,709, this condition is met when the energy spacing between levels 2 and 1 (annotated as E21) is designed to be substantially equal to the energy of the longitudinal optical (LO) phonon (˜35 meV in the case of InP-based QCLs). In this case τ2 and lower laser level population are reduced. This scheme is often called the single-phonon design.
QCL performance can be substantially improved by employing a so-called two-phonon resonance design (see, for example, U.S. Pat. No. 6,751,244) instead of the single-phonon resonance design described above. A schematic conduction band diagram for this design is shown in FIG. 3. The active region in this case is composed of at least four quantum well/barrier pairs instead of at least three for the single-phonon design. The lasing transition occurs between energy levels 4 and 3. Significantly, energy spacings E32 and E21 are both substantially equal to LO phonon energy, leading to short τ3 and τ2. Since energy spacing between the lower laser level and the lowest active region level E31 (˜70 meV) is increased by factor of two compared to E21 in case of the single-phonon resonance design (˜35 meV), the two-phonon resonance design has an advantage of reduced carrier population on the lower laser level 3 due to reduced carrier thermal backfilling for this state from the lowest active region state 1.